The calibration of an antenna system typically requires the use of an external RF (radio frequency) calibration signal source arranged to transmit a predefined signal of known form, and to receive that signal via the antenna system being calibrated.
By comparing the known form of the transmitted calibration signal to the detected form of the received calibration signal, one may calibrate the receiving antenna system according to the differences between the two. These differences are assumed to be the result, largely, of system errors and characteristics of the receiving antenna system and can be inverted to determine a correction factor or process to be applied to subsequent received signals. In this way, system errors and characteristics may be taken into account and cancelled, at least to some extent, to improve the accuracy of the antenna system.
This need for calibration is particularly important in rotating antenna systems with rotary joints, such as most radar systems.
A typical rotary joint is an electro-mechanical device that provides the required signal transfer interface between the stationary and rotating sections of a rotating antenna system. It allows radio-frequency (RF) signals to be transmitted back and/or forth between the antenna and other components of an antenna apparatus.
RF rotary joints are used in many industries. These industries include communication, satellites, aerospace and air traffic control, airborne systems, shipboard systems, ground based radar.
A rotary joint is a passive rotating transmission line that has the ability to pass RF signals with minimal degradation. However, the more RF channels required to be transmitted, generally the longer must be the rotary joint. A rotary joint can be as simple as a one-channel transmission device which is typically small (a few cm in length), or as complicated as, say, a 73 channel transmission device which is much longer (about 5 meters long).
RF rotary joints can be made of copper, bronze, aluminium, stainless steel, specialty steels, silver, and specially clad or bi-metallic alloys. Important specifications for RF rotary joints include size, length and weight. Clearly, the metallic nature of RF rotary joints renders them relatively heavy.
Transferring a radio-frequency (RF) signal across the rotary joint of a rotating antenna, such as a radar antenna array, can require large and heavy RF rotary joints close to the antenna and typically high on an antenna mast, e.g. a ship's mast. This becomes a particular problem when considering antenna arrays having many RF channels, requiring a long and heavy RF rotary joint.
The invention addresses this.